Contrast agents are mainly used to obtain images of tissues (i.e., blood vessels, tumors, etc.) and peripheral organ tissues. They are used for examining the location, size, and condition of tissues by further clarifying the brightness contrast of tumor and peripheral tissues, which have similar substituents. For distinguishing the tumor tissues from the peripheral tissues, the magnetic resonance imaging (MRI) has shown to be excellent and stable.
Many methods for examining the internal human body have been developed, and the MRI, representing the latest technology, is being increasingly used and utilized. The increase in usage is attributable to MRI's safety as compared to those of other imaging technologies. Since such methods as X-ray, CT and PET involve administrating radioactivity, which cannot be deemed completely harmless to human body, there is a disadvantage of inapplicability to patients susceptible to possible genetic mutation such as cancer patients or pregnant women. However, the MRI is a new technology, which is not restricted by radioactivity in terms of its inapplicability to certain subjects. The advantages of MRI are the facts that it shows good sensitivity to the tissues and that it does not expose patients to ionic radioactivity. Currently, MRI is being widely used in such areas as chemistry, biochemistry and medicine, and on account of its popularity, the cost of MRI has been steadily declining. Due to its short diagnostic time, the MRI is expected to gain further popularity in the future.
MRI (Magnetic Resonance Imaging) is a very advance technology, which can image brains and in vivo cellular tissues by using magnetic resonance imagining. In applying the concept that the spin of hydrogen nuclei in water, fat, etc. in vivo changes according to the applied magnetic field, the MRI is a technology which transfers the signals registered according to said changes to corresponding images. The distributions of hydrogen nuclei in water and fat are different depending on the types of tissues in vivo and also depending on the clinical conditions of the tissues. By utilizing these points and exciting the hydrogen in the area of interest, the nucleus spins are induced, and the signals thereof are measured for diagnostic purposes. The brightness of an image is affected by various variables, including the density of hydrogen, and the relaxation times (T1, T2) of excited hydrogen. In particular, the relaxation time (T1, T2) is a duration time for hydrogen to reach the original equilibrium state by way of an energy drop from the excited state (after energy absorption). These values are very important variables to the brightness contrast of an image. The smaller the values of T1 and T2, the greater the contrast of brightness, and through contrasting the brightness, shown as such, the internal tissues and organs could be distinguished for diagnostic purposes.
Meanwhile, these types of MRI images could be enhanced by using contrast agents. A contrast agent for MRI is a pharmaceutical preparation, which enhances the contrast of images by reducing the relaxation times of T1, T2, etc. of human tissues. The main types of contrast agents for MRI are products using paramagnetic or super-paramagnetic components. By using contrast agents, the signals of target organs or tissues, in part or in whole, could be amplified, or the signals of the peripheral tissues could be weakened. In this manner, the contrast of brightness could be maximized. However, due to acute toxicity of most of the paramagnetic metals, the inorganic salts of paramagnetic metals in general are not preferable as contrast agents. In order to solve this type of problems, organic chelated ligands or metal-binders are used. Organic chelated ligand, etc. form a complex with a metal, which in turn prevents free release of paramagnetic metals, and it also acts as a non-toxic carrier for paramagnetic metals, which enhances relaxation of hydrogen.
In order to use a paramagnetic metal-ligand complex as a contrast agent, there are several requirements. It needs to form a stable and hard chelate so that toxic metals are not released. Also, for easy administration to patients, it needs to have adequate water solubility and effectively enhance the relaxation rate of hydrogen ions. The efficacy thereof is generally measured in the degree of relaxation (an increase in relaxation rate per concentration (mM) of paramagnetic complex).
Gries et al. in U.S. Pat. No. 4,647,447 disclosed a complex used as a diagnostic reagent. Further, the active paramagnetic ingredient of Magnevist, which is an MRI contrast agent approved by the FDA, is a complex of diethylenetriaminepentaactic acid (DTPA) and gadolinium (III). However, the half-life period of DTPA-Gd is extremely short (about 14 minutes) so that after its administration it is rapidly washed out via urination (Hiroki Yoshikawa et al., Gazoshindan 6, 959-969 (1986)). Accordingly, with a single administration, it is difficult to make a diagnosis of various parts in the body (with respect to the site of trauma, blood vessel distribution, hemokinetic distribution, distribution amount, permeation, etc.) Further, since it is distributed non-specifically from the interior of blood vessels to the intervals of the tissue cells, there is no definite difference sometimes as between the normal tissues and the site of trauma. In these cases, a clear contrast cannot be obtained. Moreover, in the diagnostic methods using magnetic resonance imaging, the imaging time varies depending on the intensity of magnetic field of an MRI spectrometer. Hence, as for the MRI spectrometer of low magnetic field, which is widely used in general, the imaging time must be quite long. By using DTPA-Gd, which disappears from the blood vessels after a short period of time, the site of trauma cannot be examined in detail. As such, while using DTPA-Gd in diagnosis, there is a limitation according to the form of a specific site of diagnosis or a diagnostic device.
In U.S. Pat. No. 4,899,755, Lauffer and Brady disclosed a technique of synthesis of a paramagnetic metal-ligand complex for enhancing MRI with tissue-specificity to targeting tissues. This type of tissue-specific methods shows enhanced efficacy as compared to the conventional non-specific methods. In qualitative terms, a tissue-specific contrast agent confers better MR images with respect to the targeting tissues such as livers and biliaries. In quantitative terms, with a lesser amount, the tissue-specific contrast agent provides images similar to those observed by using a high-dose non-specific reagent. As such, by using a reagent specific to liver-gall, liver cancer or the malfunctions of the bilious system, which could not have been recognized (or recognized with difficulties), can now be observed.
However, the conventional paramagnetic metal-ligand complex for enhancing MRI has a problem in that it cannot be made into an appropriate pharmaceutical preparation as a contrast agent due to its low water solubility.
Meanwhile, the human blood has an osmotic pressure of 0.3 Osmol/kg-water. The conventional NMR reagents based on gadolinium (Gd) generally have a negative charge, which leads to high osmotic pressure of the water-soluble pharmaceutical preparation solutions of said reagents. For example, in case of making Gd(DTPA)2− into a pharmaceutical preparation for using it as 0.5M of N-methylglucamine salt in water, the osmotic pressure of the solution is 1.6˜2.0 Osmol/kg-water (here, DTPA is diethylenetriaminepentaacetic acid). The contrast agent administered with such a high osmotic pressure is known to cause side effects in patients.
In light of these problems of conventional MRI contrast agents, there is an increasing demand for a superior contrast agent, having a half-life period of intermediate or long-term duration in blood, which is preferably stable with appropriate water-solubility and osmotic pressure.
Meanwhile, the X-ray contrast agents now clinically used include a variety of water-soluble iodized aromatic compounds containing 3˜6 iodine atoms per molecule. These types of compounds are electrically charged (in the form of physiologically acceptable salts) or nonionic. The most popular contrast agents today are nonionic materials. Their popularity is attributable to the research finding that nonionic pharmaceutical preparations by far are more stable than ionic pharmaceutical preparations. Further, the contrast agents should take care of osmolar load to patients. In addition to water-soluble iodized pharmaceutical preparations, barium sulfates are frequently used in X-ray examination of the gastrointestinal system. A number of non-aqueous or particle pharmaceutical preparations have been proposed as non-oral X-ray contrast agents, mainly for the hepatic or lymphatic system. As examples of general particle X-ray contrast agents administered non-orally, they include suspensions of solid iodized particles, suspensions of liposomes containing aqueous iodized pharmaceutical preparations, and emulsions of iodized oils. The research and development of X-ray contrast agents has been carried out for almost 100 years, but there is a continuous demand for stabler X-ray contrast agents with superior light absorption capability.